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Full text of "Handbook Of Chemical Engineering - I"

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EVAPORATION                                         359
climates. Large quantities of salt are manufactured by this method from sea water in southern California, South America, Portugal, France, Spain and Italy. Salt plants in Germany and Austria concentrate weak brines by pumping them repeatedly over high towers made of wood and filled with thorn brushes (Gradier-werke). During the last few years, weak potash brines have been concentrated in the Western deserts by solar evaporation with good success. This process, however, requires large exposed surfaces, as the evaporation amounts to only about 1H lb. of water per square foot per day in Nevada, and about 1 Ib. in Brazil.
Evaporation by Direct Heat.Next to solar evaporation, this is probably the oldest method of concentrating liquors. Open pans with bottoms of various shapes to increase the heating surface are still used to a large extent in the chemical industries on account of their low first cost and simple construction. Sometimes straight or corrugated flues have been added to increase the heating surface, but on account of the low factor of heat transmission between hot gases and a boiling liquid, the evaporative capacity is small. Even with a well designed furnace, the evaporation will not exceed 5 lb. of water evaporated per square foot per hour, and under average conditions will not be more than 3 lb., the same as in standard boiler practice. The efficiency is low because the gases generally escape at a high temperature. One pound of coal will usually evaporate from 4 to 6 lb. of water and only rectangular shaped salt pans (grainers) which are fro^ 60 ft. long, and where the flues are arranged in four or more passes have shown an efficiency of 10 lb. of water evaporated per pound of soft coa*. oil and natural or artificial gas will give similar results to coal if the different AJJ. B.t.u. is considered. Waste gases from boilers and furnaces may be used but the evaporative effect is low and will not exceed 2 lb. per hour per square foot depending on the temperature of the gases.
It has been suggested quite frequently to combine a direct-heated tubular pan with a multiple-effect evaporator in order to save the cost of the boiler. In practically all cases these attempts have resulted in complete failures because of the difficulty of constructing and operating such an apparatus, and also for the reason that the first cost of the direct heated evaporator is much higher than the combined cost of a standard boiler and evaporator.
Evaporation by Steam.Saturated steam is by far the most efficient heating medium, and is used in the chemical and allied industries at various pressures in single- and multiple-effect evaporators, working under pressure or vacuum. Superheated steam is not suitable for evaporation of liquids in large quantities as it acts as a gas and therefore requires very large heating surfaces.
Hot water as a heating medium is used for very low and also for high temperatures. In the first case, the water is circulated through coils or jackets at low temperature under ordinary pressure to prevent burning of the liquid to be evaporated. In the second case, the water is heated under high pressure for the purpose of evaporating liquids which have a very high boiling point. The amount of water in circulation is small to prevent serious accidents in case of explosions.
Oil is used frequently instead of water under high pressure to eliminate the dangers from explosions. Both these methods of evaporation are expensive in first cost and operation, and are only applicable for special cases. Electricity as source of heat is only used in laboratory apparatus; for commercial purposes it may be used where fuel is not obtainable, and electricity can be produced by cheap waterpower.